1. Field of the Invention
The present invention relates to a parallel flat plate type plasma CVD apparatus, and particularly to a parallel flat plate type plasma CVD apparatus having a structure suitable for dry cleaning in a reaction chamber and a dry cleaning method thereof.
2. Description of the Related Art
In fabrication of various semiconductor devices or the like, as methods of forming thin films, there are a sputtering method using a sputtering phenomenon under reduced pressure, a vacuum evaporation method using an evaporation phenomenon, a plasma CVD (Chemical Vapor Deposition) method using low temperature gas decomposition by plasma, a thermal CVD method using heat decomposition of a gas, a photo-CVD method for decomposing a gas by energy of short wavelength light or ultraviolet rays, and the like. Among these, in the plasma CVD method, a gas easy to decompose and containing an element of a thin film to be formed is supplied into a reaction chamber under reduced pressure, high frequency electric power is supplied between electrodes to generate plasma, and the supplied gas is activated by the plasma, so that an objective thin film can be formed at a low temperature. This method is used for thin film formation of an amorphous silicon film, a silicon oxide film, a silicon nitride film or the like.
However, an objective thin film is adhered to not only a surface of an object to be treated, but also a wall surface of the reaction chamber, a surface of the electrode, a shielding member, and the like. As an accumulated film thickness increases, the adhered film peels off from the wall surface of the reaction chamber, the surface of the electrode, the shielding member, and the like, so that particles are produced in the reaction chamber. Thus, there occurs a problem that the produced particles adhere to the surface of the object to be treated so that the surface is polluted and the yield is lowered. Accordingly, it is necessary to remove the adhered film when the thickness of the film adhered to the wall surface of the reaction chamber, the surface of the electrode, the shielding member, and the like reaches a predetermined accumulated film thickness, or when an operating time reaches a predetermined time. As one of methods of this removing, for example, there is dry cleaning in which an etching gas is introduced into the reaction chamber and is activated by plasma similarly to the thin film formation, and the adhered film is removed by plasma etching.
An example of the dry cleaning using the plasma etching will be explained using a parallel flat plate type plasma CVD apparatus schematically shown in FIG. 1A.
A first electrode 102 as a high frequency voltage application electrode and a second electrode 103 as a ground electrode are provided in a reaction chamber 101. The reaction chamber is kept under reduced pressure by a vacuum exhaust system 110 including a turbo molecular pump 108, a dry pump 109, and the like. Heaters (not shown) are attached to the first electrode and the second electrode, and a temperature condition suitable for the dry cleaning is kept. An etching gas used for the dry cleaning is controlled by a mass flow controller 107 to have a gas flow suitable for the dry cleaning, and is supplied into the reaction chamber through a valve 106 (hereinafter, they are collectively referred to as an etching gas supply line). Then, a high frequency voltage is supplied from a high frequency power supply 104 through a matching circuit 105 to the first electrode 102 to generate plasma, and the dry cleaning by etching is carried out. Here, since the high frequency power supply is connected to the first electrode, and the second electrode is grounded, the applying voltage waveform of the high frequency voltage becomes as schematically shown in FIG. 2A. Note, time for one period is determined as t second.
In the plasma CVD apparatus as shown in FIG. 1A, at the time of thin film formation, in order to make temperature in the reaction chamber an excellent thin film formation condition, there is a case where heater temperatures of the first electrode and the second electrode are set to different values. Besides, since the high frequency voltage is supplied to the first electrode and the second electrode is grounded, the deposition mechanisms of films adhered to the wall surface of the reaction chamber, the surface of the electrode, and the like become different from one another, so that the film qualities and accumulated film thicknesses are also different. Further, also in the case where the dry cleaning by the plasma etching is carried out, by the same factor, the etching mechanisms become different among the wall surface of the reaction chamber, the surface of the electrode, and the like, so that etching speeds are also different, and removal of the adhered films is irregular.
For example, in the plasma CVD apparatus as shown in FIG. 1A, the adhered films are etched and removed in the order of FIGS. 3A, 3B and 3C. That is, the films are removed in the order of the second electrode, the wall surface of the reaction chamber from the vicinity of the second electrode to the vicinity of the first electrode, and the first electrode, that is, with directionality from the second electrode to the first electrode. It appears that this is caused since in the case of the first electrode, although a chemical etching progresses by a chemical reaction of an active radical and the adhered film, in the case of the second electrode, a small number of ions exist together with the active radical, and in addition to a chemical etching by those ions, a physical etching by a sputtering effect is also added. Since the adhered films on the second electrode, in the vicinity of the second electrode, and the like are removed in this way earlier than the first electrode, they are forced to receive plasma irradiation in the state where the surfaces are exposed, so that the damage by plasma has been serious.
Besides, since the removal of the adhered films is irregular, a time required to completely remove the adhered films becomes long, and superfluous gas, electric power and the like are consumed by that.
The constitution of the present invention is a parallel flat plate type plasma CVD apparatus and a dry cleaning method thereof, which is characterized by including a matching circuit, a first change-over switch, and a pulse amplitude modulation circuit between a high frequency power supply and a first electrode and between the high frequency power supply a second electrode, and an inverter circuit, a second change-over switch, and a wiring line for ground between the first change-over switch and the second electrode.
The dry cleaning method of the parallel flat plate type plasma CVD apparatus is such that after an etching gas is supplied into a reaction chamber, pulse amplitude modulated high frequency voltages are applied through the pulse amplitude modulation circuit to the first electrode and the second electrode. At this time, the phase of the pulse amplitude modulated high frequency voltage applied to the second electrode is shifted by 180xc2x0 with respect to the first electrode through the inverter circuit.
Besides, in accordance with irregularity in the adhesion of adhered films in the reaction chamber, that is, in accordance with irregularity in the film quality and accumulated film thickness of the adhered films in the reaction chamber, and in accordance with irregularity in plasma etching, a pulse interval of a signal wave for pulse amplitude modulation is arbitrarily selected.